Re: [RFC PATCH 3/6] mm: nommu: fix an issue on map request to /dev/zero

[Hajime Tazaki <thehajime@gmail.com>]

Hello Greg,

thank you for your time to look at this patch.

On Thu, 13 Aug 2026 21:19:47 +0900,
Greg Kroah-Hartman wrote:
> 
> On Thu, Aug 13, 2026 at 03:33:58PM +0900, Hajime Tazaki wrote:
> > Upon a private file mapping request to /dev/zero, it calls
> > kernel_read() in do_mmap_private(), getting a failure with the message
> > like: "kernel reads not supported for file /dev/zero", which is because
> > zero_fops defined in drivers/char/mem.c has both .read and .read_iter
> > definitions.
> 
> Do you actually use a no-mmu system?

yes, and wish to use in future.

I'm going to make a long story short;

nommu kernel is originally for embedded devices but I wish to use for
virtualization.

I gave a talk to explain this motivation below (which you also had a
keynote remotely).  I'm not make a summary for this talk in this
email, but am happy to explain again if needed.

https://www.netdevconf.info/0x1A/sessions/talk/improving-debuggability-of-nommu-code-with-uml.html
https://speakerdeck.com/thehajime/nommu-uml-netdev-0x1a/

> > Even fixing this issue, the map request to /dev/zero works fine without
> > errors but the allocated vma isn't marked with anonymous because
> > mmap_zero_prepare() isn't called under nommu platform, resulting
> > vma_desc_set_anonymous() isn't called either.
> > 
> > This commit fixes those issues by:
> > 1) use vfs_iter_read() instead to avoid failure at kernel_read()
> > 2) calls .mmap_prepare on private mapping in do_mmap() so that required
> >    preparations are done even in private mapping.
> > 
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> > Cc: Jan Kara <jack@suse.cz>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: "Liam R. Howlett" <liam@infradead.org>
> > Cc: Lorenzo Stoakes <ljs@kernel.org>
> > Cc: Vlastimil Babka <vbabka@kernel.org>
> > Cc: Jann Horn <jannh@google.com>
> > Cc: Pedro Falcato <pfalcato@suse.de>
> > Cc: linux-fsdevel@vger.kernel.org
> > Cc: linux-mm@kvack.org (open list:PAGE CACHE)
> > Fixes: 4d03e3cc5982 ("fs: don't allow kernel reads and writes without iter ops")
> 
> Given the age of this issue, I don't think anyone uses no-mmu systems
> anymore :(

I tend to agree; nommu systems does have less users than others.
but I believe this doesn't mean there are no users.

I should also explain more, but I found this issue via a Sashiko
review, which pointed me as a different comment, but when I tried to
reproduce that case pointed by the review, which is about the use of
check `vma->vm_file` v.s., `vma_is_anonymous(vma)`, I found that
/dev/zero is not able to map on nommu kernel.

https://sashiko.dev/#/patchset/20260710054648.924005-1-thehajime%40gmail.com
https://sashiko.dev/#/message/m2cxwvti7z.wl-thehajime%40gmail.com

The alternate path of open()=>read() of /dev/zero works fine as it
doesn't use kernel_read().


I also understand what you feel from the age of this issue (almost 6
years old), and that is exactly why I start implementing test cases in
kselftest, which is currently not able to run (even build) for nommu
platforms.  This series also includes the extension to kselftest
([4,5,6/6] patches are for that).


> > -	if (!file) {
> > +	/* private and file mapping will be marked anonymous later (do_mmap_private()).
> > +	 * and /dev/zero is marked by them at .mmap_prepare,
> > +	 * which should be _before_ this point.
> > +	 */
> 
> Wrong coding style for the comment, which is very typical of
> LLM-generated stuff.  Always rewrite the output of these tools to
> actually be sane.

this is my fault.

As I mentioned in the Assisted-by tag, I indeed used an AI review
system (which is cubic.dev), but I wrote this comment by my hand.

Because I thought this part is not easy to follow and felt that I need
additional explanation rather than just showing diff.

since checkpatch.pl doesn't complain this, I didn't carefully look at
the style of this comment block.

I will update this (as well as bunch of comments from Sashiko).

thanks,
-- Hajime